1. Field of the Invention
The present invention relates to a medical linear accelerator (LINAC). More particularly, the present invention relates to a radiation treatment beam quality assurance system for monitoring and assessing quality assurance parameters of a radiation treatment beam for a medical LINAC using image analysis methods.
2. Description of the Related Art
Medical accelerator based radiotherapy using a medical linear accelerator (LINAC) is a potentially curative treatment modality for a variety of cancers. Its effectiveness, however, is highly dependent on the radiation dose being delivered. The current standard of dose accuracy is better than +/xe2x88x925% with a geometric precision of 1 mm-5 mm, depending on the treatment site. In order to guarantee such a demanding accuracy, performance guidelines for medical LINAC beams have been established by governmental organizations, as well as professional organizations, such as the American Association of Physicists in Medicine (AAPM) Task Group-40 (TG-40). TG-40 recommends that a radiation oncology physicist perform monthly measurements of the following LINAC beam image quality assurance parameters: radiation beam symmetry, radiation beam uniformity, digital readouts of radiation field size, coincidence between the light beam localizer and the radiation field, accuracy of beam cross-hair placement, constancy of radiation field penumbra, collimator jaw angle and alignment.
Using traditional methods, such measurements are both difficult to make and time consuming. Moreover, making such measurements represents only a small portion of the overall burden of quality assurance testing. Analyzing, evaluating, and tracking the measured data represent additional burdens on the system.
Accordingly, there is a need for an improved image quality assurance system that is fast and efficient. There is also a need for a system that is capable of capturing more data with higher spatial resolution than with conventional point or linear scanner measurement techniques. There is also a corresponding need for a way to analyze, evaluate, and track the measured data.
The present invention provides an integrated measurement and analysis system, referred to herein as an image-based quality assurance (IBQA) system for providing automated quality assurance testing of a medical linear accelerator (LINAC) used in therapeutic radiation treatment. Instead of using conventional methods of beam quality verification which are error prone and time consuming, the IBQA system according to the present invention provides an efficient and robust method of quality assurance testing that is fast, reliable and objective.
In accordance with one aspect of the present invention, the IBQA system includes two parts, an imaging phantom and an integrated image analysis (IIA) system. The imaging phantom is preferably made of an opaque polystyrene composite made of two substantially square opaque plastic plates mated together by fastening screws. A middle region of the top plate is bored out to a few millimeters in depth such that when the two plates are mated to one another a chamber or slot is formed therein for receiving radiographic film.
In operation, the imaging phantom is exposed to a beam of radiation which is recorded on the radiographic film. In an alternate embodiment, an electronic portal imaging device may be used to record the beam of radiation.
The imaging phantom further includes a set of fixed reference markers, which are radio-opaque, embedded substantially flush within the top plate of the imaging phantom. The fixed markers are positioned adjacent to the chamber or slot which, when radiographed, set the orientation of the phantom, determine x-y scaling factors and measure spatial distortions. Establishing x-y scaling factors is required to correct for distortions which occur when the sampled LINAC beam image is digitized in a film scanner prior to performing an analysis. It is well known that a digitizing operation can distort scaling differently in the x and y directions. Therefore, some means of correcting for this distortion is required. The fixed disk markers serve to correct for this distortion.
The imaging phantom further includes eight rotatable radio-opaque markers for determining the degree of misalignment between the radiation field edges and a localizing light field of the LINAC.
The IIA system is configured to operate with the imaging phantom and includes hardware and software for analyzing, storing, and tracking a plurality of LINAC beam image quality assurance parameters from a sampled LINAC beam image. The software used in the IIA system is specifically tailored to the imaging phantom allowing an operator to load an image, register and analyze that image by simply clicking the mouse button twice on an IIA display screen.
The IIA system includes viewing and processing software for evaluating the sampled beam image; display means to display the measured beam image quality assurance parameters to allow comparison with baseline beam quality assurance parameters to determine whether one or more parameters are outside a prescribed threshold; and software for producing quantitative reports in accordance with government mandated regulations (e.g., American Association of Physicists in Medicine TG-40 guidelines); an integrated database which stores the measured parameters to: establish baseline and tolerance tables for all beam image quality assurance parameters for each accelerator modality and energy; record and retrieve beam quality parameter results for trend analysis and data mining for most AAPM TG-40 beam quality assurance parameters; select specific protocols, such as from xe2x80x9cProtocol and procedure for quality assurance of linear acceleratorsxe2x80x9d by Chris Constantinou, for defining the beam image quality assurance parameters; and generate customized reports responsive to government mandated regulations.
A method consistent with the present invention for employing the inventive IBQA system includes the steps of: setting up the imaging phantom including the steps of: leveling the imaging phantom; aligning light field cross hairs of a LINAC with cross hairs on the imaging phantom; and aligning marks on rotatable disks with a light field edge of the LINAC""s light localizer. Subsequent to setting up the imaging phantom, exposing a sheet of radiation sensitive film contained within the imaging phantom to a beam of energy to obtain a sampled image; digitizing the sampled image; detecting the radiation field edges in the sampled image; searching the sampled digitized image for the image of a plurality of fixed and rotatable radio-opaque disk markers; measuring a plurality of beam quality assurance parameters from the sampled image; displaying the measured beam quality assurance parameters along with a set of baseline beam quality assurance parameters highlighting those measured beam quality assurance parameters which fall outside an acceptable range as defined by a corresponding baseline beam quality parameter; and storing the measured beam quality assurance parameters in a database for providing customized reports and for tracking the parameters over time.
The IBQA system disclosed will have great value to test operators and other agents responsible for analyzing and reporting quality test results of a medical LINAC. The time required to make beam image quality measurements is significantly reduced while providing greater accuracy an higher reliability than conventional techniques of radiation beam measurement and quality testing.